The present invention is directed to a flaw detector and to a method of flaw detection. In particular, the invention is directed to a sliding slope technique for detecting container flaws. Such flaws include checks, cracks, chips and line overs which are often found in containers such as glass bottles. A check is a split in the glass bottle which arises from uneven cooling. A crack is a split due to mechanical forces. A chip is a missing or crumbled piece of glass. A line over is a groove in the top surface or finish of the bottle.
Typically, flaw detection is performed as the container is rotated. An optical system including a light source and photosensor scans a portion of the container surface. The level of the photosensor output is proportional to the changing light transmitted through or reflected from the surface. A flaw such as a check or crack directs increased or brilliant light to the photosensor. The increased light is sensed as a "light spot". A flaw such as a chip or line over attenuates the light incident on the photosensor. The attenuated light is sensed as a "dark spot". Signal processing circuitry is usually employed to amplify and shape the photosensor output in preparation for comparison to a predetermined threshold level ("amplitude comparison" technique). If the amplitude of the processed signal (or an average amplitude) exceeds the amplitude of the threshold level, the container is regarded as a reject. A rejected container is ejected from the line.
Using an amplitude comparison technique to detect flaws poses difficult problems. Changes in light incident on the photosensor due to normal container surface characteristics, such as threads, seams, seeds, neck rings and blisters, interfere with and even resemble changes arising from flaws such as checks and cracks. Distinguishing between such normal surface characteristics and true flaws is troublesome.
One solution is to mask the field of view of the photosensor. By this technique, the field of view is limited to a relatively small section of the container surface void of threads, seams or other such problem areas. Masking the photosensor, however, complicates set-up and is often unduly time consuming. Additionally, since photosensor masking drastically reduces the field of view of the photosensor, it is necessary to provide additional sensors to fully examine the container.
Detection of flaws such as chips on the inside finish of a glass bottle and line overs cutting across the top finish have been especially difficult to detect. As mentioned previously, these flaws are sensed as "dark spots" in the bottle finish and may be indistinguishable from ambient light.
Conventional optical inspection systems are described in U.S. Pat. Nos. 4,002,823 (Van Oosterhout) issued Jan. 11, 1977 and 4,136,930 (Gomm et al.) issued Jan. 30, 1979. U.S. Pat. No. 4,002,823 discloses a system for detecting defects in articles such as glassware. A video camera receives semi-diffused light passing through the glassware. The video signal is representative of the spatial rate of change of the refraction characteristics of the glassware. The signal is filtered and peak-detected to discriminate between normal marks and defects. An instantaneous amplitude value is taken and compared to a running average ("average amplitude comparison" technique). A defect pulse is generated when the instantaneous amplitude exceeds the running average.
U.S. Pat. No. 4,136,930 discloses a system for detecting foreign particles in the liquid contents of a bottle. The bottle is first viewed by a video camera which "memorizes" its optical characteristics. The bottle contents are then agitated to cause any foreign particles to shift position. The bottle is then viewed by a second camera. Movement of the foreign material changes the optical characteristics viewed by the camera. Differences in amplitude between adjacent video pulses indicate the presence of foreign material.
Other optical inspection schemes are disclosed in U.S. Pat. Nos. 3,886,356, 3,900,265 or 3,997,780.
U.S. Pat. No. 3,886,356 discloses an optical scanner for identifying defects in a transparent item by measuring the light transmitting qualities of the item. Circuitry responsive to change in signal amplitude is employed to detect the presence of defects.
U.S. Pat. No. 3,900,265 discloses a laser scanner for flaw detection. The scanner responds to changes in the amplitude of light reflected from the surface being inspected.
U.S. Pat. No. 3,997,780 discloses a label orientation inspection system. A vidicon camera views a pattern generated by an array of fiber optic cables focused on a bottle. The pattern is compared with a reference pattern to determine proper alignment of the label.